Many Gamemasters never go beyond Expert. They feel no need. They already know how to make a Role-Playing Game fun and exciting, and they see no reason to search for something deeper.
And they are right in one sense - the tools of Mastery aren't about making a game more fun and exciting. Instead, the tools of Mastery are about making the game memorable.
Any game session has the potential for being memorable, and for lots of different reasons - after all, it's a group conversation, and there are many things that can make a conversation memorable.
But a Master GM makes the game itself memorable.
And the manner in which the Master GM does so is through the use of Technique. A Master GM is in command of every aspect of the tale and the conversation and the game, and makes full use of everything at their command in order to bring something special to the game - something called Artistry.
Think of a movie that you enjoyed, but didn't really like. It was fun, but it had no other real appeal to you. Putting that into the terms of our lessons, that would be a 'Great' movie.
Now, pretend that that film also had had good dialog. (Conversation, in other words.) In our terms, that would be an 'Exceptional' film. Its level of quality would now be far above most movies. But it would still be lacking in several areas, keeping it from being 'Expert.'
Now, sharpen this film's craft. Pretend that every element that went into the craft of the film was done well. Now, in our terms, you would have an 'Expert' film on your hands.

And yet, as well-crafted and witty and fun as this film now is, it is still a movie lacking in lasting appeal. It is an excellent film, but one you are likely to forget about in only a few years.